What a whorl is
Hair follicles grow at an angle, and at the crown they change direction around a central point, so the hair fans out in a spiral. Dermatologists describe single and double whorls at the crown as normal patterns, and a whorl that makes hair stand up, at the crown or along the front hairline, is what most people call a cowlick. At the centre of a whorl the hair parts by itself, so a small spot of scalp can show, more so with straight, fine, light or short hair. A whorl has been there since childhood, so it shows in old photos too. With two whorls, the hair can also part between them, which is easy to mistake for thinning.
Based on: Samlaska et al. 1989
What crown thinning looks like
In male-pattern hair loss the crown, or vertex, can thin into a roughly round area that widens over time. Norwood described a stage III vertex for thinning mainly at the crown while the front recedes no further than stage III, and in the later stages the bare crown and front join up. The BASP classification grades density on the crown separately, for men and women alike. Within the area, follicles shrink, so the hairs turn finer, shorter and lighter before they stop, and scalp shows through a zone much wider than a whorl's centre, with no sharp edge. In women, thinning more often spreads across the top with a widening part, which the Ludwig scale describes.
Based on: Norwood 1975; Lee et al. 2007; Ludwig 1977
How to tell them apart
Seen from above, a whorl is a point and thinning is an area. Look at where the scalp shows: in a whorl, the hair spirals out from one centre and lies dense just around it; in thinning, scalp shows across a wider patch, even where the hair does not part. Compare a few hairs from the crown with hairs from the sides or back: similar thickness is reassuring, while many fine, short hairs suggest the follicles are shrinking. Then check the trend. A whorl looks the same in photos taken years apart, while thinning grows, slowly but steadily, like a bald spot that spreads. A partner, a hairdresser or a second mirror helps, because nobody sees their own crown well.
Based on: American Academy of Dermatology
Lighting traps that fake a bald spot
Crown photos are easy to get wrong. Spotlights straight overhead, as in many bathrooms, lifts and changing rooms, shine down between the hairs and bounce off the scalp, and flash does the same at close range. Wet hair clumps into strands and opens gaps, as do oils, gels and freshly washed fine hair. A short cut leaves less hair to cover the whorl. Dark hair on pale skin exaggerates every gap, while light hair on fair skin can hide real thinning. A phone held at arm's length often sees the crown from an angle. Reviews of hair measurement rate standardized photographs among the most useful tools, precisely because the set-up is the same every time.
Based on: Chamberlain & Dawber 2003; Canfield 1996
When a crown change needs a doctor
Slow thinning at the crown is common and can be discussed with a dermatologist whenever you want to know your options. See one soon if a patch appears within days or weeks, if it is smooth and round, if the scalp there is scaly, sore, burning or itchy, or if you see small black dots, which are broken stubs of hair. Round patches can be alopecia areata, and scaly patches can be a fungal infection that needs prescription treatment. Central centrifugal cicatricial alopecia, most common in women of African descent, usually starts as a small round patch at the crown that grows outward and can scar for good, so early assessment matters.
Based on: American Academy of Dermatology; NHS